I'm pleasantly surprised that somebody finally replied again.
In the days since anybody but me has commented, I changed both the Forest and Darkness Medallions.
Forest was just +1VP on my first go-around. It didn't have as much flavor as the rest and didn't win every time, so I added the +1 Card just before posting. After a couple games, it was glaringly obvious that it was too powerful. I think I have a solution: +1VP Gain a Copper and put it into your hand.
Darkness was much more broken than I expected. It has had very little testing, but I tried changing up the bonuses to "fix" it. I rotated the bonuses, so now you have to discard Action cards to get +Cards, Treasure cards to get +Actions, and Victory cards give the coin and buy. This also makes buying one at the start +2 Actions +1 Buy +(1) instead of a Market; probably better for a 4. I was also thinking of only starting you off by drawing 1 card instead of 2, but that feels too weak at first glance.
I really liked how the Light and Spirit Medallions seemed to line up with other cards while still being interesting enough.
Time feels OK for 1 more coin than Storeroom while replacing +1 Buy with +1 Action, but it might not be enough.
I haven't thought of a good way to make Water weaker, but it didn't feel like the strongest, so I haven't put in that much thought. Similar situation with Shadow, but I'm not sure that it's too strong yet.
Fire is my favorite at the moment, so I'm glad that one doesn't need improved. XD